In case of a frequency deviation in a grid, most electrical generators comprised of electrical machines directly connected to the grid provide a natural inertial response compensating for the occurring frequency deviation. For example, when the frequency declines, the generator shaft slows, releasing kinetic energy that is converted to electric power, which partially compensates for the lack of energy that causes the frequency decline. When the frequency increases, the generator shaft speed increases, causing the generator to absorb energy from the power system, thereby partly compensating for the surplus of energy that caused the frequency increase. Conventional electrical energy generators, like a coal plant or a nuclear plant, provide inertial response according to the inherent characteristics of the turbine and generator used for the power generation.
Converter-controlled equipment, like wind turbine and solar PV, do not provide any response to abnormal frequency unless this capability is programmed into the controls. A requirement to provide this capability is often stipulated by grid operators. As more and more renewable energy power plants like wind turbines using full inverters are providing this ancillary service as transient frequency response to the grid, the grid operators may need more flexibility in selecting the magnitude of the response to such frequency drops. At this time, in case of a frequency drop in a grid, a renewable energy power plant like a wind turbine may be constrained to provide the maximum transient frequency response it is able to deliver to the grid at the threshold frequency. With the increasing number of wind turbines as renewable energy power plants, this may lead to an overcompensation of the frequency deviations in a grid, which may lead to further de-stabilizing the grid frequency
Therefore, there may be a need for an improved reaction possibility for a wind turbine as well as for an entire wind turbine power plant fleet, in case of a detected frequency deviation in a grid, in order to provide an appropriate assistance for a stabilization of the grid frequency.